Conventionally, a passive sampler that is exposed to the atmosphere and performs collection by natural diffusion, has been employed in collection of a substance-to-be-collected such as formaldehyde floating in the atmosphere (Patent Document 1). This passive sampler sets a substance-to-be-collected concentration always to zero at a collecting inner surface and maintains a constant concentration gradient with an inlet port that is a collecting outer surface contacting a collecting atmosphere whose concentration is higher than that at the collecting inner surface, by means of an absorbing agent or collecting agent provided within the passive sampler, thereby utilizing a movement diffusion phenomenon of substance concentration to collect the substance-to-be-collected.
A collecting rate per unit area of the passive sampler, that is, a unit collecting speed J is determined by a concentration gradient (C/L) and a molecular diffusion coefficient D [cm2/sec] of a diffusing substance, and this can be expressed by the so-called Fick's first law of diffusion equation (J=D× C/L). Now, in Fick's first law of diffusion equation, C expresses a concentration [μg/ml] in a periphery, and L expresses a diffusion length [cm], that is, a length from the inlet port of the passive sampler to a collecting surface. Moreover, as shown in equation (SR=A×D×C/L), a total collecting speed SR of the passive sampler can be obtained by multiplying the collecting speed per unit area J (J=D×C/L) by a collecting area A. Generally, this is called a sampling rate SR, and is known as a characteristic collecting speed with respect to a certain specific substance-to-be-collected of the passive sampler. This sampling rate SR is expressed by a dimensional unit such as [μg/(ppm×min)], and this is known to make it possible to find a concentration C′ of the substance-to-be-collected in the atmosphere, from a value of the sampling rate SR and a value of a collection amount M [μg] and collection time t [min] obtained from a post-collection analysis result, based on equation (C′=M/(SR×t)).
However, there is a problem that because collection is performed by natural diffusion, the sampling rate SR of the passive sampler does not indicate a constant value due to an influence of wind speed on-site, and the sampling rate SR fluctuates significantly particularly in the case of a low wind speed. This is conceivably because a peripheral wind speed of the passive sampler decreasing causes a diffusion concentration gradient to extend from the passive sampler whereby a concentration gradient gets created outside the passive sampler and a value of the diffusion length L greatly changes. As shown in FIG. 8, such “extension of diffusion field” is prominent at a wind speed of under 0.5 m/sec, particularly at a low wind speed of 0.1 m/sec or less. Now, FIG. 8 is a graph showing results of verification carried out by the applicant of the present application using the passive sampler that shows a relationship between collection amount of the passive sampler and wind speed under a steady environment, and, in order to clarify a reference, the vertical axis in FIG. 8, that is, the collection amount of the passive sampler is expressed by a ratio [%] to a collection amount obtained by an active method where collection is performed using a pump (collection amount of passive sampler/collection amount by active method).
In order to prevent such “extension of diffusion field”, it is conceivable to, for example, increase a film thickness or reduce an opening diameter or opening ratio of the passive sampler to secure a larger diffusion length L and thereby apparently reduce the extension. However, it is impossible in principle to set the extension to 0, and in an actual measurement environment, the wind speed is constantly fluctuating and the diffusion length L also ends up fluctuating based on the wind speed fluctuation, hence it is close to impossible to predict and make constant an apparent diffusion length L in the passive sampler. As a result, there is a problem that in the case of accurately measuring concentration of a substance-to-be-collected in the atmosphere using the passive sampler, it is necessary to employ a general calculation method that finds the sampling rate SR as an actual value according to a measurement site or compares with an active method using the likes of a pump to find the sampling rate SR beforehand as a representative value, and employ this sampling rate SR, and the method is complex and moreover an inferior method whose accuracy is lower than that of the active method.
As a collecting apparatus capable of solving such problems, the applicant of the present application proposes, for example, a collecting apparatus 300 shown in FIG. 9 (Patent Document 2). As shown in FIG. 9, this collecting apparatus 300 is configured capable of housing a passive sampler 302 within a housing 310, and is configured such that a wind speed can be forcibly generated within the housing 310 by a flow means such as a fan 322. Such a collecting apparatus 300 enables an influence of wind speed on-site to be eliminated, hence makes it possible to achieve stable collection of the substance-to-be-collected.